1. Field of the Invention
The present invention relates to the field of mobile digital communications, and more particularly, cellular communications.
2. Prior Art
Phase locked loop techniques (Costas loops) for tracking carrier phase and frequency do not work very well in the mobile channel because the phase change due to Rayleigh fading is so rapid that very wide loop bandwidths are required to track phase. The bandwidth required is so wide that performance in noise is so poor as to render the approach useless for all but the most slow moving vehicles.
A class of nonlinear estimation algorithms is described to estimate the unknown phase of a carrier which is fully modulated by m-ary phase shift keying (PSK) modulation in "Nonlinear Estimation of PSK-Modulated Carrier Phase with Application to Burst Digital Transmission", IEEE Transactions on Information Theory, vol. IT-29, No. 4, July 1983. This technique has been used in burst digital transmission modems for military applications.